Low exess air operation of multipleburner residual-fuel-fired furnaces

ABSTRACT

1,115,889. Controlling burners. ESSO RESEARCH &amp; ENGINEERING CO. 18 July, 1966 [2 Aug., 1965], No. 32194/66. Addition to 1,080,069. Heading F4T. A process for the control of low excess air combustion in a multiple-burner furnace comprises (a) adjusting the air-fuel feed to a first burner while analyzing the concentration of a flue gas component whose concentration is proportional to the level of excess air operation of this burner, thereby establishing the stoichiometric point of the burner; (b) adjusting the first burner to the desired low excess air level and (c) sequentially repeating (a) and (b) for each of the other burners. Any further adjustment to the fuel-air feed indicated by analyzing the flue gases is made to all the burners simultaneously. The analysis is carried out on either the carbon monoxide or carbon dioxide component of the flue gases. Step (b) is carried out either by a trial-and-error technique or by determining the burner flame temperature and adjusting the air level until the temperature has altered by 25‹ F. which will provide for the required 3% excess air level.

29, 1966 J. E. GERRARD ETAL 3,288,199

LOW EXCESS AIR OPERATION OF MULTIPLE-BURNER RES IDUAL-FUEL-FIRED FURNACES Filed Aug. 2, 1965 FLUE GAS ANALYZER 5 5 5 W5 FUEL 7. 7. 7. MANIFOLD LEVEL- FUEL SUPPLY 8 AIR MANlFOLD SUPPLY PA TENT ATTORNEY Patented Nov. 29, 1966 3,288,199 LOW EXCESS AIR OPERATHQN F MULTIILE- BURNER RESlDUAL-FUEL-FIRED FURNACES John E. Gerrard, ltiatawan, Charles W. Siegmund, Morris Plains, and Wladimir Philippoif, Cranford, N.J., assignors to 13550 Research and Engineering Company, a corporation of Delaware Filed Aug. 2, 1965, Ser. No. 476,662 4 Glaims. (Cl. 158-1175) The present invention relates to multiple-burner residual-fuel-fired furnaces and boilers. In general, it concerns a process and apparatus for establishing and maintaining optimum combustion at a low level of excess air in a multiple-burner furnace or boiler. In particular, it provides a process for achieving this optimum combustion by achieving the proper adjustment of the individual burners of such multipleburner systems.

Multiple-burner residual-fuel-fired furnaces or boilers are commonly used in firing power plants. Such furnaces or boilers may have a dozen burners, be as tall as an eight story building and consume up to 400 barrels of fuel oil per hour. The residual fuel oils employed in such burners contain ash (i.e. metallic contaminants that are both non-volatile and non-combustible) and sulfur. Ash is troublesome in nearly all types of combustion equipment. The most common problem is deposit formation or slagging which reduces heat transfer, etc. Sulfur is found in varying amounts and chemical combinations in all grades of residual fuel oil. Some heavy fuels from high sulfur crudes contain as much as 5 wt. percent sulfur, while others from low sulfur crudes may have less than 1 wt. percent. Typical is a No. 6 oil having an average sulfur level of 2.5 wt. percent. When sulfurcontaining fuels are burned, part of the sulfur is converted to sulfur trioxide which in turn causes sulfuric acid corrosion.

It is well known that the operation of residual-fuel-fired systems is substantially improved in terms of reducing superheater slagging and corrosion of both superheaters and air preheaters by carrying out the combustion at levels of low excess air only slightly higher than stoichiometric, e.g. 0.01-5% excess. This is an effective method of optimizing combustion, but in the past has been limited in application because smoke formation and poor combustion usually result before the low excess air combustion is achieved. One of the problems associated with achieving this low excess air combustion is assuring that the individual burners of a multiple-burner system are each operating at the same level of excess air. For instance, if half the burners in a multiple-burner system operate at 6% excess air, and the remainder operate at stoichiometric conditions, the flue gas analysis of the stack gases (which gases, of course, are a composite of the gases from all the individual burners) for the percent oxygen will show the overall combustion to be at 3% excess air (i.e. optimum combustion conditions). However, conditions are actually poor in terms of smoke and corrosion, since the burners operating at or slightly below stoichiometric conditions will produce dense smoke, and those operating at 6% excess air will tend to cause superheater and air preheater corrosion.

According to the present invention, there is provided a process and apparatus which assure that the individual burners of a multiple-burner system are each operating at substantially the same level of excess air and thus afford a practical means of realizing the inherent benefits of low excess air combustion.

In the present invention, analysis of components in the common flue gas enables the establishment for each individual burner of a substantially stoichiometric combustion point. The same level of excess air combustion for each individual burner is obtained by backing off from this point. Each burner is treated individually in this manner, i.e. first establishing a substantially stoichiometric combustion point, and second backing off. This process is conveniently referred to as tuning the burners. The tuning is repeated until each individual burner is operating at the same level of excess air. The process of continuously tuning the burners involves an iteration in the mathematical sense that successive trials home in on the solution if the system is inherently convergent which this is. After the burners are tuned, if the level of excess air combustion is other than that which is desired, appropriate adjustment of all burners simultaneously to achieve the desired level is easily effected.

Either of two basic flue gas analyses can be used in the first step of the process of the present invention. In the first, the analysis is performed on the combustibles in the flue gas. The combustibles are substantially carbon monoxide, which is the most sensitive component in the combustion system as regards rapid concentration change at the stoichiometric point. In the second, the analysis is performed on any component of the combustion system which peaks in concentration near the stoichiometric point. Carbon dioxide is one such component which peaks in concentration near the stoichiometric point.

When carbon monoxide analysis is used to tune the burners, the following procedure is followed:

Assuming a multiple-burner furnace to be in a state wherein the individual burners are operating at various levels of excess air, all Well above stoichiometric, the concentration of carbon monoxide in the common flue gas is close to zero.

In practice, most furnaces are generally operated in such a state because of the smoke and air pollution problems which result if any burners are not operating on excess air. systematically reducing the air supply to only the first burner increases the carbon monoxide level Within the common flue gas very sharply as this first burner approaches the stoichiometric point. At some arbitrary level shortly after the beginning of the rapid rise in carbon monoxide concentration in the common flue gas as detected by a combustible analyzer, it is established that the burner in question is operating at the substantially stoichiometric combustion point. It is not important that the carbon monoxide level in the common flue gas correspond to the exact stoichiometric point for the particular burner. However, it is important that each subsequent burner be brought to the same common flue gas carbon monoxide level, i.e. substantially stoichiometric point. After the first burner has been brought to the substantially stoichiometric point the burner is backed off this level in order to achieve the desired level of low excess air combustion, e.g. about 23% excess air.

The backing off is accomplished by increasing the air and/ or decreasing the fuel feed to the particular burner in question. The extent to which the bumer should be backed off can in general be determined by either one of two methods. The first, and preferred, method involves a trial-and-error technique. In this technique, the air and/ or fuel supply valve to the individual burner is turned, i.e. backed off a measurable displacement.

In the second method, a flame temperature measuring device is used to determine the extent to which the burner should be backed off. For example, sheathed refractory-metal thermocouples'provide a means of determining flame temperature. Other devices such as closedtube photopyrometers may also be used. Whatever the device used, it is known that flame temperature varies about 6 to 10 Fahrenheit degrees per percent excess air.

It follows, therefore, that each burner should be backed off in terms of temperature to the same extent e.g. 20 to 30 F. This second method requires the use of some means for measuring flame temperature. In the past great difiiculty has been encountered in providing suitable devices which would continue to function without failure under the high temperature conditions which exist within the furnaces. In this second method, the device is subjected to these high temperatures only once, i.e. during initial calibration. For example, a thermocouple or other device is used initially to establish the extent to which the burner should be backed off. The result is then converted into a fixed displacement on the individual air (or fuel) control valve for the burner in question and the device is removed from the furnace. It is apparent from the above that the first method described does not require the use of any temperature measuring device.

It is,.of course, conceivable, and in fact, very likely, that the excess air level established in the first burner will be upset by the redistribution of fuel and air taking place during the rest of the tuning. This is easily counteracted by reinitiating the whole sequence when the last burner is tuned. It is apparent then, that the burners are iterated into balance.

After the burners are tuned, the level of excess air combustion is determined by flue gas analysis of the stack gas for the percent oxygen. If the results of this analysis indicate that the level is other than that which is desired, appropriate adjustment of all burners simultaneously to achieve the desired level is easily effected.

The individual components of the apparatus employed in carrying out the process of the present invention when carbon monoxide analysis is used are Well known and, per se, form no part of the present invention. Suitable apparatus would include a carbon monoxide or combustibles analyzer a level-sensing device on the output from the analyzer which activates the air and/or fuel control valves on the respective feeds to the individual burners.

The invention is further understood by reference to the accompanying drawing which shows a schematic diagram of one embodiment of the same.

With reference to the drawing there is shown a furnace 1 equipped with four burners 2. Air and fuel are supplied to each burner 2 through the respective control valves 3. Fuel is supplied through supply means 5 to each control valve 3 from a fuel manifold 4. Air is supplied through supply means 7 to each control valve 3 from an air manifold 6. Control valve 8 regulates the air/ fuel ratio to the fuel manifold 4 and air manifold 6. A flue gas analyzer 9 is located in the furnace stack 10 and analyzes the mixture of flue gases from the four burners 2. Signals from the level-sensing device 11 on the output from flue gas analyzer 9 are conducted through suitable conduit means 12 to activate air and/ or feed control valves on the respective feeds to the individual burners. Each control valve 3 and control valve 8 are shown as single valves, however, two separate valves, one on the fuel supply line and one on the air supply line, can also be employed at any or all of these points in the system.

When carbon dioxide or some other component of the combustion system which peaks in concentration around the stoichiometric point is used to tune the burners the following procedure is used:

Assuming a multiple-burner furnace to be in a state wherein each of the individual burners are operating at various levels of excess air all well above stoichiometric, there is some fixed level of carbon dioxide in the furnace stack. Reduction of the air level to the first burner increases the carbon dioxide concentration in the flue gases from the flame of the first burner and therefore in the overall stack analysis. As the air to the first burner is progressively reduced, the carbon dioxide in the stack gases continues to rise until the stoichiometric point of 'the first burner is reached, after which time the level begins decreasing. In this manner, the stoichiometric point for the first burner has been fixed and a backing olf procedure similar to that described above is followed to establish the desired level of low excess air combustion for the first burner. Each individual burner is treated in this manner, as with the carbon monoxide analysis.

The individual components of the apparatus employed in practicing the process of the present invention when a component which peaks in concentration around the stoichiometric point is used are well known to those skilled in the art, and per se, form no part of the present invention. This apparatus would include a peak component analyzer, e.g. 'a carbon dioxide analyzer and a peaksensing device on the output from the analyzer which actuates air and/ or fuel control valves on the respective feeds to the individual burners.

By means of the present invention there is provided a relatively uncomplex, inexpensive process by which low excess air combustion at each individual burner of a multiple-burner furnace can be automatically controlled. It is readily apparent that this process may be employed wherever multiple-burner residual-fuel-fired furnaces are found, for example, in power-generating plants, both on land and at sea. While a general description has been given and a preferred embodiment of the present invention has been described, it is to be understood that various modifications and adaptions thereof to different uses may be made without departing from the spirit of the invention and the scope thereof as described by the following claims.

What is claimed is:

1. A process for the control of low excess air combustion in a multiple-burner furnace during operation of the burners thereof which comprises (a) adjusting the air-fuel feed to a first burner of said furnace, while analyzing the concentration of a flue gas component whose concentration in the flue gas is proportional to the level of excess air operation of said first burner, thereby establishing essentially the stoichiometric point of said first burner; (b) backing off said first burner to the desired level of low excess air combustion and (c) sequentially repeating (a) and (b) on each burner of said multiple-burner furnace, thereby establishing substantially the same desired level of combustion at each burner of said furnace.

2. A process according to claim 1 wherein said flue gas component is carbon monoxide.

3. A process according to claim 1 wherein said flue gas component is carbon dioxide.

4. A process according to claim 1 wherein said desired level of low excess air combustion is determined by measuring the flame temperature at each of said burners.

References Cited by the Examiner UNITED STATES PATENTS 1,645,350 10/1927 Reineke 23615 2,666,584 1/1954 Kliever 236l5 3,015,357 1/1962 Bain et al 158117.5

JAMES W. WESTHAVER, Primary Examiner. 

1. A PROCESS FOR THE CONTROL OF LOW EXCESS AIR COMBUSTION IN A MULTIPLE-BURNER FURNACE DURING OPERATION OF THE BURNERS THEREOF WHICH COMPRISES (A) ADJUSTING THE AIR-FUEL FEED TO A FIRST BURNER OF SAID FURNACE, WHILE ANALYZING THE CONCENTRATION OF A FLUE GAS COMPONENT WHOSE CONCENTRATION IN THE FLUE GAS IS PROPORTIONAL TO THE LEVEL OF EXCEES AIR OPERATION OF SAID FIRST BURNER, THEREBY ESTABLISHING ESSENTIALLY THE STOICHIOMETRIC POINT 